The vessels were originally intended as "submarine monitors", but their purpose had been changed before detailed design began. M1 was fitted with a 12-inch (305mm) gun which was intended for use against surface ships in preference to torpedoes, the argument being that, "No case is known of a ship-of-war being torpedoed when under way at a range outside of 1000 yards".[1]

Sectioned model in the Science Museum, showing the 12 in (300 mm) turret

Although the gun had an effective range of 15,000 yards (14 km), it was normally fired using a simple bead sight at periscope depth with only the barrel above the water. It was important for the submarine's gun to sink or disable the target with the first shot, because the gun could only be loaded on the surface.

She was 295 feet 9 inches (90.14 m) long, displaced 1,950 long tons (1,980 t) submerged and operated out of Portsmouth. She was launched on 9 July 1917, but was not involved in active service in the First World War.

In 1923, water leaking into the barrel of the gun resulted in extensive damage to the muzzle when it was fired.[2] She sank with all 69 hands on 12 November 1925 while on an exercise in the English Channel. A Swedish ship, SS Vidar, struck the submerged M1 and sank her in 70 m of water. The collision tore the gun from the hull and water flooded the interior through the open loading hole. The crew members appear to have tried to escape by flooding the interior and opening the escape hatch, but their bodies were never found.